18 INTRODUCTION 



Organs are made up of various parts called tissues. The 

 leaf has an outer, lifeless, protective covering or tissue 

 beneath which are the tissues that manufacture food. Other 

 tissues conduct the manufactured food to the stem. Tissues 

 are composed of cells having a similar function. In the light 

 of our present knowledge the cell is the ultimate unit of 

 structure of living organisms. 



The history of the term cell is a curious one and affords a 

 good illustration of the manner in which our scientific con- 

 ceptions gradually become modified and improved as our 

 knowledge increases. The term cell was unknown before 

 the invention of the compound microscope. Robert Hooke 

 observed for the first time the cellular structure of plants, 

 of which he published his account in 1665. He examined a 

 thin piece of bottle cork and named the little openings 

 separated by firm walls, cells. Robert Hooke's idea that a 

 cell consisted of a small cavity filled with air and surrounded 

 by a wall persisted until 1846, when wn Mohl gave the 

 . name protoplasm to the slimy contents discovered by him in 

 the cells of living tissue. In time it became evident that this 

 protoplasm was the vital constituent of the cell and that it 

 was identical with minute organisms without cell walls, such 

 as the amoeba. The term cell is therefore applied at the 

 present time to masses of .protoplasm with or without a wall. 



Protoplasm, the living part of the cell, possesses several 

 functions, chief among which are the following: 



1. Irritability. 



2. Growth. 



3. Reproduction. 



1. That living protoplasm is irritable is shown by its 

 reaction to varying external conditions, such as changes in 

 temperature and in intensity of light. 



2, The power to grow or increase in size is possessed by all 



